If your window frames are meant to have tapered head screws (like "sheet rock" screw heads), then you might want to use them again. They would just be a "flat head wood screw."
Usually, going through thin metal on the way to wood, I would figure on using a "round head screw" which has a head with a flat bottom (where it touches the metal) and a rounded top. That's because a tapered (flat) head could wallow out the metal in its attempt to countersink itself.
Do other Trilliums have flat head screws holding the windows
in? (i.e. flat top, tapered bottom to the head).
PS: Meant to add that all these types (and much, much more) are available in stainless. You can order from www.mcmaster.com
They have a great online catalog, or you can call them.
has a good stainless fastener selection too.
Note that I'm used to ordering in the US, not Canada, so that bears checking.
Depending on where you are on the island, Sidney or Victoria might have them - check at marine stores. I haven't been there in a few years, so I don't know who is in business there now.